Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Display devices are typically controlled with either a passive-matrix control employing electronic circuitry external to the substrate or an active-matrix control employing electronic circuitry formed directly on the substrate. Organic Light-Emitting Diode (OLED) display devices have been fabricated with active-matrix (AM) driving circuitry in order to produce high performance displays. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066. Active-matrix circuitry is commonly achieved by forming thin-film transistors (TFTs) over a substrate and employing a separate circuit to control each light-emitting pixel in the display.
In an active-matrix device, active control elements comprise thin-films of semiconductor material formed over a substrate, for example amorphous or poly-crystalline silicon, and distributed over a flat-panel display substrate. Typically, each display sub-pixel is controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each light-emitting element typically employs an independent control electrode and a common electrode. Control of the light-emitting elements is typically provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
The TFTs of an active-matrix display device may be composed of a thin layer (usually 100-400 nm) of a semiconductor such as amorphous silicon or polysilicon. The properties of such thin-film semiconductors are, however, often not sufficient for constructing a high-quality display. Amorphous silicon, for example, may be unstable in that its threshold voltage (Vth) and carrier mobility may shift over extended periods of use. Polysilicon often has a large degree of variability across the substrate in threshold voltage (Vth) and carrier mobility due to the crystallization process. Since OLED devices operate by current injection, variability in the TFTs can result in variability of the luminance of the OLED pixels and degrade the visual quality of the display. Compensation schemes, such as adding additional TFT circuitry in each pixel, have been proposed to compensate for TFT variability; however, such compensation may add complexity, which can negatively impact yield, cost, and/or reduce the OLED emission area. Furthermore, as thin-film transistor fabrication processes are applied to larger substrates, such as are often used for large flat-panel television applications, the variability and process cost may increase.
One approach to avoiding these issues with thin-film transistors is instead to fabricate conventional transistors in a semiconductor substrate and then transfer these transistors onto a display substrate. U.S. Patent Application Publication No. 2006/0055864 A1 by Matsumura et al. teaches a method for the assembly of a display using semiconductor integrated circuits (ICs) affixed within the display for controlling pixel elements, where the embedded transistors in the ICs replace the normal functions performed by the TFTs of prior-art displays. Matsumura teaches that the semiconductor substrate should be thinned, for example by polishing, to a thickness of between 20 micrometers to 100 micrometers. The substrate is then diced into smaller pieces containing the integrated circuits, hereafter referred to as ‘chiplets’. Matsumura teaches a method of cutting the semiconductor substrate, for example by etching, sandblasting, laser-beam machining, or dicing. Matsumura also teaches a pick-up method where the chiplets are selectively picked up using a vacuum chuck system with vacuum holes corresponding to a desired pitch. The chiplets are then transferred to a display substrate where they are embedded in a thick thermoplastic resin.
The process taught by Matsumura, however, may have several disadvantages. For example, semiconductor substrates are typically 500 micrometers to 700 micrometers in thickness; therefore, thinning the substrate in this fashion may be difficult and, at low thicknesses, the crystalline substrate may be very fragile and may be easily broken. Therefore the chiplets may be very thick, at least 20 micrometers according to Matsumura. The thick chiplets of Matsumura may result in substantial topography across the substrate, which may make the subsequent deposition and patterning of metal layers over the chiplets more difficult. For example, Matsumura describes concave deformations as one undesirable effect.
Another disadvantage of the process taught by Matsumura is that the surface area of the chiplets must typically be large enough to be picked up by the vacuum-hole fixture. As a result, the chiplets may have a length and a width that are larger than the minimum size of the vacuum hole.
Employing an alternative control technique, Matsumura et al. describe crystalline silicon substrates used for driving LCD displays in U.S. Patent Application 2006/0055864. This application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown.
In either of the above methods, there is a chance that chiplets deposited on a substrate may be misplaced, that the chiplets may be faulty, and/or that the chiplets may fail to be placed entirely. Moreover, subsequent substrate processing steps can damage the chiplets, inadvertently relocate the chiplets, and/or fail to interconnect chiplets properly. Such process failures can render the substrate completely or partially inoperable.